FirstDigest

Cisco: Configuring Compressed Real-Time Protocols

Calin — Thu, 20 Nov 2008 13:32:32 +0000

I had lately to configure compressed real-time protocols (CRTP) over a Frame-relay link.
I thought that it will be good to make a tutorial about how to configure this on the Serial interfaces (with HDLC or PPP encapsulation) and on the Frame-relay interface. Another type of interface supporting this is and ISDN interface, but the configuration there is the same like in the Serial interface case.
If you were asking why I don’t configure this on a Ethernet interface, well this is because CRTP is not supported on the Ethernet interfaces. This was just an explanation for those of you who didn’t knew this. The role of CRTP is to improve communication over low bandwidth links like Serial interfaces, Frame-relay or ISDN, and that’s why is not supported on Ethernet links (who’s speed is starting at 10 Mbps).

In the following tutorial I will configure CRTP on a Serial interface with PPP encapsulation (I chose this randomly as is the same with HDLC and ISDN) and Frame-relay interface, limit the number of header compression connections and check CRTP with “show…” commands. Since the purpose of this tutorial is CRTP the connection will no be active.

See the tutorial below:

Cisco: How-to limit HTTP traffic on weekdays during working hours

Calin — Wed, 19 Nov 2008 13:54:58 +0000

Some time ago a person asked me to set the HTTP traffic to 256 kbits during weekdays from 8:00 - 16:00, to limit “the fun” in the office while other are working. In theory I’m against this type of policy, because if you have a team of network engineers and they have to access cisco.com in the same time for information and other stuff like IOS download, then this will take a lot of time. Of course if you see that the productivity is going down because 80% of the traffic is to YouTube site (nothing against them, just picked randomly), then such measures are welcomed.

Since the discussion below is beyond the scope of this tutorial and represent only a personal opinion, let me tell you what I’ll show in the tutorial below. On Fa0/0 (out connection) we will limit the HTTP responses sent out to 256kbps from Monday to Friday between 8:00 and 16:00

Please see the tutorial below:

Cisco: Multilink PPP over Frame Relay (MLPoFR)

Calin — Tue, 18 Nov 2008 23:20:44 +0000

In this tutorial I propose to show something that is not very used these days, or at least not every day, but which can be tricky if you don’t know how to approach this type of configuration. To understand this, I assume that you know the basics about PPP, FR and Multilink. I will make a short summary here but I will not go into details:

PPP or Point-to-Point protocol is used to establish direct connection between two network points. It can provide authentication, encryption privacy and compression.
FR or Frame-Relay is a telecommunication service used mostly on the WAN side towards your provider or carrier and it relay on frames for data transmission.
Multilink is used for bundle together 2 or more channels / circuits for communication improvement.

Here we will use these 3 technology to create something called MLPoFR. For security we will use authentication. Please download here the topology. Please be aware that in the topology you cannot see actually 2 links there (it a limitation of GNS3), but trust me the links are there. To be more convenient and quick the R1 of the topology is preconfigured.

Please see the tutorial below:

Cisco: Deny false information routing injection into OSPF domain

Calin — Mon, 17 Nov 2008 20:45:36 +0000

In a well controlled environment, false information routing should not reach your OSPF domain, as network engineer take care what to advertise and what not into OSPF. But there are cases when you have to deal with 3rd party companies somehow, and you want to be sure that nothing in injected by mistake into your domain. Also this can be a task for CCIE RS lab exam.

And since I specified that this can be an exam task, let take some “DO NOT USE” rule and we have to accomplish the task above without using the command “ip ospf authentication message-digest”. Download the used topology here. R1 from the topology is pre-configured. The OSPF timers have been reconfigured to hello 1 second and dead interval 5 seconds, not to wait “forever” until it rebuilds the adjacency.

Please see the tutorial below:

Interface macro command on a Cisco switch

Calin — Sun, 16 Nov 2008 11:42:24 +0000

From the beginning let me tell you that I don’t see very useful this command, as I prefer to use “interface range…” syntax, but since I saw it as a requirement in one of the task for CCIE RS lab exam, and maybe somebody will find it usable in real environment, I said I should put it here in a tutorial.

As many of you already know, you can control a range of interfaces by typing the command “interface range Fa0/1 - 6″ (for example), but there is another way to do this by using the interface macro style. For those how are beginners, this interface range or macro syntax spare you from typing 6 commands under 6 interfaces (stick to the example above), but issue only one command under interface range or macro.

Please see the tutorial below:

Limit traffic on a Cisco switch L2 port with minimal configuration

Calin — Fri, 14 Nov 2008 13:38:18 +0000

Let’s say that somebody (or some task in a test) ask you to limit the inbound traffic on a switch Layer 2 port by using minimal configuration possible. I must say that in the first steps I failed this task miserable, but actually is very simple to do it.

I will use a plain layer 2 Cisco 2950 switch for this task. I observed that I could not implement this on a Cisco 3500XL. I don’t know if the IOS image was wrong, but I didn’t investigate too much in that area as I cannot stand 3500XL switches and they are actually pretty old piece of hardware.

No topology is needed for this as I will only show how to do it and not testing it with real traffic. I will do testing later when I’ll have some more time, or you can do it on your own.

See the tutorial below:

Cisco hidden tool: test crash

Calin — Thu, 13 Nov 2008 11:57:10 +0000

This is pretty old trick, so maybe you already know it, but for the rest of you, it can be interresting.

You just had a crash on you Cisco hardware and you have the logs, but don’t know exactly what caused this crash. You tried to have the device crash again, to compare the logs maybe you’ll find the cause, but your device won’t crash (of course, it will crash only when don’t want that to happen).

There is a hidden Cisco command: “test crash”. This can help you if you are lucky enough to have the real crash exactly like one of those you can test with “test crash” command.

Note: As you can see in the previous posts I do my tutorials based mostly on Dynamips, when it is possible. This is not the case. So, please do not use Dynamips for this test, as it will go into errors since the device is simulated and not a real one.

Please see the tutorial below:

Frame-Relay: PVC test with ping to own interface

Calin — Thu, 13 Nov 2008 08:28:05 +0000

Let’s say that you have a link configured with Frame-Relay and from time to time you observe that the link is having some strange problems. Since you cannot be sure that the link is provided 100% error free you want to do your own tests. You want to see if the Frame-Relay between R0 and R1 is correctly configured. In order to verify this you want to set up the R0 to test the PVC to R1, by sending traffic to its own IP address.

Please download the topology here. The Frame-Relay between R0 and R1 is already configured.

See the tutorial below:

Cisco hidden tools: Testing TCP connection

Calin — Wed, 12 Nov 2008 11:36:17 +0000

Sometimes you have a connection between 2 routers that you want to test with something more than a ping. Cisco devices have a hidden command which is actually a very powerful testing tool for your TCP connection.

The command is “ttcp” and you will not find it in the the default list of commands of a Cisco device. So, even if you use help ( “?” mark at the router prompt) this command will not be showed to you.

For our testing we will use the same topology as in the previous posts. If you do not have it please download it here. Since this is a point-to-point TCP connection testing we will not use any fancy routing protocol or other networking protocols.

See the tutorial below:

How to force Loopack interface to be advertised with a /24 netmask

Calin — Tue, 11 Nov 2008 22:42:35 +0000

Many of you already know that usually when you advertise a Loopback interface into OSPF it is advertised with /32 netmask (one IP address) even if the IP address under Loopback interface is with a /24 netmask. If you checked my previous post, you’ll see at the end that I’m right, and the Lo0 address is advertised with /32 netmask.

But what if you have to advertised as a /24 subnet (task require or some testing) ? There is “dirty little trick” to do it. I will use the same topology and design like in the post before. If you do no have the topology please download it here.

Watch the movie: